Waveguide duplexers



Nov. 26, 1957 H. J. RIBLET WAVEGUIDE DUPLEXERS 3 Sheets-Sheet 1 OriginalFiled April 28, 1948 7C RECEIVE/2 TENNA IGNAL R. E POWER IN FROMTRANSMITTER A NTENNA /NVENTO/? -/-/E/v/2v J. 17/51. ET BY 5% g ,M

ATTORNEY Nov. 26, 1957 J, RIBLET 2,814,784

WAVEGUIDE DUPLEXERS Original Filed April 28, 1948 3 Sheets$heet 2 Fla. 5

TO OSCILLATOR NON REFLECTIVE TERM/NAT! ON HEN/9V J. P/BL ET ATTOPNE'VNOV. 26, J, ET

WAVEGUIDE DUPLEXERS Original Filed April 28, 1948 3 Sheets-Sheet 5 Fla/O FIG.

FREQUENCY IN MEGAC'YCLES /NVEN7D/? HEN/2v J. R/BLET ATTORNEY UnitedStates Patent WAVEGUIDE DUPLEXERS Henry J. Riblet, Wellesley, Mass.,assignorto Raytheon Manufacturing Company, a corporation er DelawareOriginal application April 28, 1948, Serial No. 23,822. Divided and thisapplication August 25, 1952, Serial 13 Claims. (Cl. 333-10 This is adivision of my copending application, Serial No. 23,822, filed April 28,1948, now Patent No.

In radar systems, there is usually a single antenna for i use in bothtransmitting and receiving. To prevent the strong bursts of energy thatare available during transmission from entering the receiver, anelectronic switch is provided which has the effect of decoupling thereceiver from the system and coupling the antenna to the trans mitteralone during transmission. When transmission ceases, the receiver iseffectively recoupled to the system. This switch, generally known as aduplexen or a transmit-receive device, or simply, a T-R, includes as anelement a spark gap, properly located in the tran'smission lines thatinterconnect the receiver and transmitter with the antenna, and adjustedto break down when power of the order of that available fromthe't'ransmitter is flowing through the transmission line between thetransmitter and the antenna, but not when the lower order power such asis available during reception is present. It is now customary to providea certain amount of auxiliary unidirectional potential, for exampleabout 300 volts, as a keep alive in the vicinity of the spark gap, tomaintain an amount of residual ionization of the gases surrounding thegap and thereby assure firing of the gap during transmission. Duringreception, some of the received energy attempts to enter thetransmitter, 45

which is at that time poorly matched impedance-Wise to the transmissionline, and is mostly reflected. To prevent a this energy from disturbingthe system, a second spark gap device is employed, properly located inthe transmission line system with respect to the first. This seconddevice functions during receiving to decouple the transmitter from thesystem, and is known as an anti 1 R or R-T. The two spark gap devicesare intended to have the effect of decoupling the transmitter from-thereceiver at all times.

It is an object of this invention to provide an improved duplexer whichis small, compact, and rugged in construction.

It is a further object to provide an improved duplexer which reduces thenumber of discharge tubes required in radar systems and generallysimplifies the construction of such systems.

It is a still further object to provide such a dnplexer which is readilyinstalled in a radar system with known installation techniques.

The invention consistsessentially of a four-terminal network composed oftwo waveguides directionally coupled together, for example, at a commonwall by means ofslo'ts in said wall, and is based on the discovery thatthe directional coupler character of the network is not destroyed whenthe coupling slots are short-circuited, for example,

2,814,784 Patented Nov. 26, 1957 by an ionic discharge. The network isconnected in a radar system by coupling three of the terminals thereofto the radio frequency power source or transmitter, the antenna, and thereceiver, respectively, and providing the fourth terminal with anon-reflective termination. Other and further advantages andcharacteristics of the invention, as well as the details of certainparticular embodiments thereof, will become apparent from the discussionthat follows, reference being made to the accompanying drawings,wherein:

Fig. sis a side-sectional view of a duplexer like that of' Fig. 1showing the manner of connecting it into a 'radarsys'tem; I

Fig. 4illu'strates another form of duplexer constructed in accordancewith the invention;

Fig. 5 illustrates a preferred form of the duplexer shown in Fig. 1;

Fig. 6 is a top plan view of the duplexer shown in -1.

Fig. 7 is a view taken on the line 7--7 of Fig. 6; Fig; 8. is. a topplan view of a modification of the "duplexe'r of Fig. 5;

9 is a top plan view of another modification of "the duplexer shown inFig. 5;

, Fig. 10 is a view along line 1010 in Fig. 9; and Fig. 11 is aperformance curve illustrating a certain broad band characteristic ofthe invention. Referring now to Fig. 1, two parallel waveguides 10 and11' are joined together at a common wide wall 12. A pair ofrectangularly crossed slots 13 and 14 is cut in this emamofi wall, in aregion to one side of the g tudinal center line thereof. At each of thefour pointswhere the walls of the slots intersect, the material of' thewall 12 is continued in the form of a pointed extension 15 inthedirection of a diagonally oppositely disposed" intersection point. Theextensions 15 extend toward-the geometric centerof the slotconfiguration, and all terminate just short thereof, so that their endsare all very near to. but do not touche'ach other. This construction isshown more clearly in Fig. 2. The slots 13 and 14 are each resonant tothe mid-band frequency of the operative frequency band, and consequentlytend to erect practically percent transfer from one waveguide to theother ofpower at that frequency. The upper waveguide 10 is sealed atboth sides of the slot arrangement with suitable gas impervious butelectromagnetic wave pervious windows 16, 17 made of glass, mica, orthe. like, anda similar window 18, shown in dotted line in Fig 1-butmore clearly in Fig. 3, is provided in the other waveguide 11, mountedonthe common wall 12 and covering the slots 1-3 and 14. There is thusprovided a sealed chamber 19 in the upper waveguide 10, and theextensions 15 are in-this chamber. The chamber 19 is partiallyevacuated, and may contain anysuitable gas, for example hydrogen orargon, and water-vapor at a pressure-below tha't ofpthe atmosphere.

The four. terminals of the duplexer of Fig. l are denominated 1,2, '3,and 4,-the ends of the lower, unsealed 'waveguide 11 being terminals 1and 3, respectively, and 5 the ends of the u'pp'er, se'aled waveguide 10being terminal's 2 and 4, respectively. This duplexer is connected witlra; complete system as shown in Fig. 3. Terminal 1 is connected to thetransmitter, or source of radio frequency power (not shown), which isusually the mago netron. Terminal 2 is connected to the receiver (notshown). Terminal 3 is connected to the antenna (not shown). Terminal 4'is' connected to a non-reflective terw shown) which does not reflectenergy. In Fig. 3, the

window 18 at the slots is shown in cross section, and it should be notedthat this window is spaced away from the 14 and the extensions 15. Thepurpose of this will be presently explained.

As has already been set forth, the present duplexer is essentially adirectional coupler. Considering first the from the antenna, this powerenters terminal .3, in the direction of line 22, and proceeds to theleft along the lower waveguide 11. When the received signal arrives atthe resonant slots 13 and 14, it is directionally coupled over into theupper waveguide 11, and proceeds through terminal 2 to the receiver (notshown). A small fraction of the received signal fails to be coupled overinto the upper waveguide 10, and, as indicated by a dotted line 22',proceeds toward terminal 1 and the transmitter.

Here however there is a marked impedance mismatch, due to the fact thatthe transmitter is dormant, and this stray signal power is reflectedback toward terminal 3. On arriving at the slots 13 and 14, however, thestray signal power is directionally coupled into the upper waveguide 10,and therein to the non-reflective termination 21, where it is completelyabsorbed for all practical purposes.

, This action of the present duplex rendersunnecessary the special antiT-R circuits that have heretofore been employed in radar systems, for,with the present duplexer, the transmitter is effectively decoupled fromthe receiver.

Considering now the relatively high radio frequency power that isavailable from the transmitter, this power enters the. duplexer atterminal 1 and proceeds to the right along the lower waveguide 11 asindicated by line 23. When this power arrives at the slots 13 and 14, ittends normally to proceed through the slots into the upper waveguide andtoward terminal 4 and the non-reflective termination 21. However, thevoltage produced across the slots is so great that the gaps between theends of the antenna through terminal 3. A small amount .of transmitterpower is coupled into the upper waveguide 10, however, and proceedstoward the non-reflective termination 21. I have discovered and verifiedexperimentally that the directional coupler nature of the present deviceis not destroyed by the firing of the gaps between the ends of theextensions 15, and that the portion of the transmitter power that iscoupled through the slots 13 and 14 during firing of the gaps is coupleddirectionally, as indicated by the dashed line 23', as though the gapswere not fired. In other words, that the present duplexer has high leveldirectivity as well as low level directivity is an experimental factwhich distinguishes the present duplexer in performance fromconventional balanced duplexers. The nonreflective termination 21absorbs practically all of the stray transmitter power that is coupledinto the upper waveguide 10, so that only a very small amount of suchstray power is reflected therefrom. The small amount of this power thatis reflected proceeds to the receiver, as indicated by the dot-dashedline 23", since the gaps are still fired and it cannot be coupled backin any appreciable quantity to the transmitter.

The voltage level of the transmitter power that eventually finds its wayto the receiver is so low that it cannot burn out the crystal detectorthat is ordinarily found wall material in the immediate vicinity oftheslots 13 and relatively low power that is available as receivedsignal 1 there. Thus the present duplexer combines in one device thefunctions of the T-R and the anti T-R circuits of the known art.

In general and as will be discussed in detail in connection with Fig.11, the amount of received signal power that is not coupled into thereceiver is no greater than 2 db of total power furnished at terminal 3over a band of frequencies of the order of 500 megacycles wide at 9300me. mean operating frequency and may be as low as 0.2 db thereof in anarrower band. Thus within the wider band, at least 63 percent of thereceived signal power arrives at the receiver. On the other hand, thetransmitter power that is coupled out of the lower waveguide 11 into theupper waveguide 10 is at a level approximately 40 db below that in thelower waveguide 11, so that nearly percent of the transmitter power isfurnished to the antenna. The fraction of the transmitter power that isfurnished to the receiver by reflection from the practicallyreflectionless termination 21 is at the still lower level ofapproximately 60 db below that of the original power in the lowerwaveguide 11, and this small fraction cannot damage the receiver crystaleven when energy at a 30 kilowatt level is furnished by the transmitter.

While the crossed arrangement of the two slots 13 and 14 is preferred,as will be explained below, the invention may be practiced withdirectional couplers of other forms, such as the directional couplershown and described in my copending application Serial No. 784,277,filed November 5, 1947. Fig. 4 shows a slot arrangement in accordancewith the disclosure of said copending application. Thus the common widewall 12 may be provided with one or more pairs of individual mutuallyperpendicular slots 26 and 27. The transversely directed slot 26 iscentered at the longitudinal center line of the wall 12, and thelongitudinally directed slot 27 is centered to one side thereof, so thatwhen the excited waveguide is excited in the fundamental mode, each willbe in a position of advantage for coupling energy, all as is explainedin said copending application. The present arrangement differs from thatof said copending application, however, in that the slots are resonantto the mid-band frequency of the operating frequency band, for, in thepresent invention, it is desired to secure, as nearly as possible, 100percent coupling from one waveguide to the other. Each slot is providedwith confronting projections 28 disposed in the long walls thereof sothat they originate at points of high potential difference, whichfunction as discharge electrodes in a low pressure atmosphere, as do theextensions 15.

When the slots 26, 27, and discharge electrodes 28 are all enclosed in aproper low pressure gaseous atmosphere, as in Fig. 1, the directionalcoupler in accordance with Fig. 4 may be employed in place of thecrossed slot arrangement of Fig. 1 to provide a duplexer which behavesbasically like that of Fig. 1. It is possible, however, that one of theslots 26 or 27 of a pair will fire prior to the other during the T-Rfunction, and, if that happens, the directional coupler action will beimpaired and the output signal pulse at the antenna will not be sharpand distinct, but may be irregular, with some power lost through one ofthe slots into the termination 21 or perhaps even into the receiver,where damage might result to the crystal. Hence, one reason forpreferring the crossed slot configuration shown in Fig. 1 is that,because the gaps of the two slots 13 and 14 are practically coextensive,the firing of one slot necessarily fires the other. In this way, thedirectional coupler behaves the same each time the gap is fired. Thecrossed slot configuration is set somewhat to one side of thelongitudinal center line in the common wide wall 12 so that each slotwill by itself transfer the same amount of power from one waveguide tothe other.

Referring now to Figs. 5, 6, and 7, the embodiment of the inventionthere shown employstwo waveguides 30 each other and joined together in acommon wide wall "preferably maintained as short as possible.

5 section 32. The junction may be made by removing a portion of,"for'example, the lower wide wall of the top waveguide 31 and solderingtogether the 'two waveguides so that a portion"32 of the upper wide wallof the lower waveguide 30 is common to both waveguides. A crossed slotconfiguration similar to that of Fig. 1, made of two slots 33 and 34, isprovided in one corner of the common wall area 32, so that it is to oneside of the longitudinal center line of each waveguide. The lowerwaveguide 3c is sealed at 'both endsiwith the window 16 and thenon-reflecting termination 21, and a sealing window 38, corresponding tothe window 18 in Figs. 1 and 3, is provided in the upper waveguidecovering the Slts33 and 34. A gap 36 is provided among four extensions35, which correspond to'the extensions 15 in Fig. l. The covering window38 is. preferably arched as shown in Fig. 7 so that sparking across thegap 36 will not burn it. As will be readily appreciated, the lowerwaveguide 30 of Fig. corresponds to the upper waveguide 10 of Fig. 1,and the upper waveguide 31 of Fig. 5 corresponds to the lower waveguide11 of Fig. 1. The various terminals 1,2, 3, and 4 of the dupleXer shownin Fig. 5 are denominated the same as the corresponding terminals inFig. 1, and are intended to be connected in a system in the same manneras shown in Fig. 3.

The crossed waveguide configuration of Figs. 5, 6, and

7 provides an arrangementwherein equal powers are transferred bythe twoslots at all frequencies in the operative frequency band, therebyenhancing the bandwidth and overall performance of the device.

Referring now to Fig. 8, the embodiment there shown employs areplaceable slot and gap element 40, which is mounted in the common Wallsection '32. This element may be soldered or screw-threadedly engaged inthe wall 32, as desired, and consists of a rim portion 41 and equallyspaced around the circumference thereof four radially inwardlyprojecting arms 42, which extend toward the center to provide a gap 43.The arms define a-pair of somewhat wing-shaped crossed slots 44 and 45,which together have theappearance of a four-leaf clover. In fact,-theslots 13 and 14 or 34 and35 may also be rounded at the outerends'to-provide a similar appearance. The crossed-slot arrangement ofthe invenand 38 and a'suitable matched load or non-reflectivetermination like termination 21 are provided as set forth above.

The embodiment of the invention that is shown in Figs. 9 and 10 iselectrically the same as those of Figs. 1,

5, and 8, but provides a different mechanical structure wherein the gapis adjustable and the electrodes aremore easily renewed. Two waveguides50 and 51 are disposed at right angles to each other, with wide wallsconfronting, and a smallspace 52 between them. Each waveguide isprovided with an aperture 53, 54, respectively, in the wide wallconfronting the other. The apertures are collinear and each is near anedge of its wall, so that, as shown in the top plan view of Fig. 9, theyare disposed in a corner of the square region where one waveguide 51 isdirectly above the other 50. This disposition is the same as that of theslots in the embodiments shown in'Figs. 5 and 8. A tube 55 ofelectrically conductive material connects the two apertures. This tubeis in effect a short length of waveguide, and is The tube is providedwith four radially inwardly directed screws 56 which approach but do notmeet at the center. The

space between their inner endspr'ovides a spark .gap '57.

The lower'waveguide 50 .is sealed at one end with the non-reflectingtermination 21 .or .the like and .at the otherend with the window 16(not shown). The .aperture'54 of the upper waveguide 51 is closed insidethat waveguide with a window 58. The spark gap 57 is thus completelysealed off from theatmosphere and can be surrounded withgas at .a lowpressure. The screws 56 provide for adjustment of the gap and can be.re-

placed. The threaded fit of the screws into the tube 55 must be suchthat low pressure can be-m'aintained around the spark gap. This can beassured by providing a solder seal (not 'shown) after the gap '57 isadjusted, thus been built and tested, none has been provided with akeep-alive potential, and yetnone has been known tohold off firing.

Inthe so-callcd X-band, which is in the region of 3 centimeterwavelengths,-the duplexer accordingto Figs. 5 a'nd'8 occupies a'spacewhich -can be made as small as one cubic inch. This small spaceincludesboth'the'T-R and the anti T-R equipment, whichis a'vast savingin Space over the prior knownarrangements. As has been pointed out above.in the'discussion-of Fig. 3, the duplexer of the present-inventionisactually a directional coupler atboth highandlowlevels, with apractically non-existent amount of loss at high levels, and thedirectionalcoupler featureathigh levels provides for protection vof thereceiver. For example, the high levelloss-due-to'the duplexer hasbeenfoundon some samples to be from 0.1 to 0.3 idb. Atlow=levels,'however, that is, during receiving, the duplexer'is -adirectional coupler inthe ordinary sense, the slots 'beingin the unfiredcondition, and thereis a loss of energy "due to the insertion thereofbetween the antenna and theftreceiver. Fig. 11 shows the amount of'theinsertionloss for atypical duplexer'operating in the X-band. The curve60 shows that over a wide :band, from 8900'to greater than 9400megacycles per second, the insertion loss'was never greater than 2.1 db.In: the band from 9-180 to 9375 megacycles, the loss was.0;5 db oriless.

Fnrther'measurements that have'been made indicate that the recoverytime-of the present duplexer isat least as good'as'that of prior knownduplexers. In other charac-teristics,-s'uch as leakage power,iit islikewise-as suitable =as other duplexers. In its lower losses at bothhigh andlow levels,- its savings in space, cost, and weight,'and in manyother features that have been pointed. out-above, it is vastlysupe'riorto .the prior known duplexers.

Duplexers constructed inaccordance with the' present invention may takemany physical forms, and no attempt has been 'madeherein to illustratemore than but'a few examples.

Accordingly, it is intended that the claims that follow shall not belimited by the herein described details, but only by the prior art. Inthe claims thatfollow,

which, when there exist waves travellingin both directions in the mainline, delivers to one end of theauxiliary line a voltage which islargely a function of .the amplitude of the wavegOing inone-preferreddirection in themain line, and relatively independent of the wave goingin the opposite direction in the main line. Also, in the claims thatfollow, the word annulus is to be understood as defining either the rim41 of Fig. 8, the tubular eyelet 55 of Figs. 9 and 10, or any equivalentclosure element, of whatever contour, that may serve as a boundinganchorage -for the radially extending electrode elements whose innerends form the described spark gap or gaps, and whose lateral surfacescombine with the inner periphery of the annulus to form the describedenergy-admitting passage-ways.

What is claimed is:

1. A duplexer comprising a pair of conduits for directingelectromagnetic wave energy, an energy coupling means including anannular member providing a directional wave conductive passage betweensaid conduits and a plurality of electrode elements extending radiallyinward from the peripheral wall of said annular member toward the centerthereof and terminating short of said center to form a spark gaptherebetween, said electrode elements combining with said annular memberto form energy-admitting passage-ways whose transverse width exceedssubstantially the thickness of said electrode elements.

2. A duplexer comprising a pair of conduits for direct ingelectromagnetic wave energy, an energy coupling means including anannular member providing a directional wave conductive passage betweensaid conduits and a plurality of electrode elements extending radiallyinward from the peripheral wall of said annular member toward the centerthereof and terminating short of said center to form a spark gaptherebetween, said electrode elements combining with said annular memberto form energyadmitting passage-ways of clover-leaf contour whosetransverse width exceeds substantially the thickness of said electrodeelements.

3. A duplexer comprising a pair of conduits for directingelectromagnetic wave energy, an energy coupling means including anannular member providing a directional wave conductive passage betweensaid conduits and a plurality of electrode elements extending radiallyinward from the peripheral wall of said annular member toward the centerthereof and terminating short of said center to form a spark gaptherebetween, said electrode elements combining with said annular memberto form energy-ad mitting passage-ways whose transverse width exceedssubstantially the thickness of said electrode elements, each of saidelectrode elements having its radially extending longitudinal axisdisposed in parallelism with the longitudinal axis of one of said wavedirecting conduits.

4. A duplexer comprising a pair of conduits for directingelectromagnetic wave energy, an energy coupling means including anannular member providing a directional wave conductive passage betweensaid conduits and a plurality of electrode elements extending radiallyinward from the peripheral wall of said annular member toward the centerthereof and terminating short of said center to form a spark gaptherebetween, said electrode elements combining with said annular memberto form energyadmitting passage-ways of clover-leaf contour whosetransverse width exceeds substantially the thickness of said electrodeelements, each of said electrode elements having its radially extendinglongitudinal axis disposed in parallelism with the longitudinal axis ofone of said wave directing conduits.

5. A duplexer adapted to be used with ultra-high frequencyelectromagnetic wave energy comprising a pair of wave guides each havingtwo terminals and having a common side wall, a circular aperture in saidcommon wall intermediate said terminals, an annular member adapted tofit into said aperture, a plurality of elongated electrodes mounted onsaid annular member and extending radially inwardly with their free endsconfronting to provide a spark gap therebetween and with their. lateralsurfaces combining with said annular member to form energy adt1) mittingpassages whose transverse width exceeds substantially the thickness ofsaid electrodes.

6. A duplexer adapted to be used with ultra-high frequencyelectromagnetic wave energy comprising a pair of waveguides each havingtwo terminals and having a common side wall, a circular aperture in saidcommon wall intermediate said terminals, an annular member adapted tofit into said aperture, a plurality of elongated electrodes mounted onsaid annular member and extending radially inwardly with their free endsconfronting to provide a spark gap therebetween, and with their lateralsurfaces combining with said annular member to form directionalenergy-admitting passages whose transverse width exceeds substantiallythe thickness of said electrodes.

7. A duplexer adapted to be used with ultra-high frequencyelectromagnetic wave energy comprising a pair of waveguides each havingtwo terminals and having a common side wall, a circular aperture in saidcommon wall intermediate said terminals, an annular member adapted tofit into said aperture, four elongated electrodes mounted on saidannular member and extending radially inwardly with their free endsconfronting to provide a spark gap between them, and with their lateralsurfaces cooperating with said annular member to form energyadmittingpassages of clover-leaf contour, and means supporting said annulus fromsaid wall in said aperture, said electrodes defining a pair of crossedslots resonant to said wave energy, said slots being disposed todirectionally couple said waveguides, said electrodes being sodimensioned that said spark gap breaks down when the coupled energyexceeds a prescribed value thereby effectively closing said slots.

8. A duplexer adapted to be used with ultra-high frequencyelectromagnetic wave energy comprising a pair of waveguides each havingtwo terminals, and havinga common side wall, a circular aperture in saidcommon wall intermediate said terminals, an annular member adapted tofit into said aperture, a plurality of elongated electrodes mounted onsaid annular member and extending radially inwardly with their free endsconfronting to provide a spark gap between them, and with their lateralsurfaces cooperating with said annular member to form energy-admittingpassage-ways of clover-leaf contour, means supporting said annularmember from said wall in said aperture, said electrodes defining a wavepassage through said aperture which is disposed to directionally couplesaid waveguides, and being so dimensioned that said spark gap breaksdown when the coupled energy exceeds a predetermined value therebyefiectively closing said passage, and means providing ahermetically-sealed cell about said annular member and electrodes.

9. A duplexer to be used with ultra-high frequency electromagentic waveenergy comprising a pair of waveguides having confronting spaced-apartside walls, each of said waveguides having two terminals, a thirdwaveguide connected between said side walls in directional energycoupling relationship between said waveguides of said pair, andelectrode means combining with said third wave guide to formenergy-admitting passage-ways of clover-leaf contour.

10. A duplexer adapted to be used with ultra-high frequencyelectromagnetic wave energy comprising a pair of waveguides havingconfronting spaced-apart side walls, each of said waveguides having twoterminals, a third waveguide connected between said side walls indirectional energy coupling relationship between said waveguides of saidpair, and electrode means combining with said third waveguide to formenergy-admitting passageways of relatively wide extent, said electrodemeans having confronting end surfaces forming a gap of relatively narrowextent for emergency shunting of said relatively wide passage-ways.

11. A duplexer adapted to be used with ultra-high frequencyelectromagnetic wave energy comprising a pair of waveguides havingconfronting spaced-apart side walls, each of said waveguides having twoterminals, a third waveguide connected between said pair of waveguidesin directional energy coupling relationship between said waveguides ofsaid pair, said third waveguide being disposed at a region intermediatesaid terminals, and electrode means extending transversely into saidthird waveguide with inner ends confronting to provide a spark gap ofrelatively narrow extent, said electrode means 00- operating with saidthird waveguide to define at least two wave passage-ways of relativelywide extent.

12. A duplexer adapted to be used with ultra-high frequencyelectromagnetic wave energy comprising a pair of waveguides havingconfronting spaced-apart side walls, each of said waveguides having twoterminals, a third waveguide connected between said pair of waveguidesin directional energy coupling relationship between said waveguides ofsaid pair, said third waveguide being disposed at a region intermediatesaid terminals, and four elongated electrodes extending transverselyinto said third waveguide with inner ends confronting to provide a sparkgap of relatively small area, said electrodes cooperating with saidthird waveguide to define at least two wave passage-ways of relativelylarge area.

13. A duplexer adapted to be used with ultra-high frequencyelectromagnetic wave energy comprising a pair of waveguides havingconfronting spaced-apart side walls,

each of said waveguides having two terminals, a third waveguideconnected between said pair of waveguides in directional energy couplingrelationship between said waveguides of said pair, said third waveguidebeing disposed at a region intermediate said terminals, electrode meansextending transversely into said third waveguide with inner endsconfronting to provide a spark gap, said electrode means being radiallymovable with respect to said third waveguide, and cooperating with saidthird waveguide to define wave passage-ways of clover-leaf contour.

References Cited in the file of this patent UNITED STATES PATENTS2,153,728 Southworth Apr. 11, 1939 2,407,069- Fiske Sept. 3, 19462,432,093 Fox Dec. 9, 1947 2,473,274 Bradley -a June 14, 1949 2,479,650Tiley Aug. 23, 1949 2,602,859 Moreno July 8, 1952 2,606,248 Dicke Aug.5, 1952 2,644,139 Hunter June 30, 1953 FOREIGN PATENTS 592,224 GreatBrita-in Sept. 11, 1947

